1. Field of the Invention
The present invention relates to a printed circuit board unit comprising a printed circuit board and an electronic component such as a chip mounted on the surface of the printed circuit board through solders.
2. Description of the Prior Art
Soldering is often employed to mount electronic components such as semiconductor devices or bare chips on a printed circuit board in a multi-chip module (MCM) or a semiconductor package. Soldering is also employed to mount electronic components such as the MCMs on a printed circuit board of a larger size, namely, a motherboard. When the individual bare chip or MCM is to be exchanged in the MCM or motherboard, solders must be broken or removed to release bonding between input/output terminals on the bare chip or the MCM and input/output pads arranged on the printed circuit board. For example, an exchanging operation is designed to detach the older bare chip or MCM from the printed circuit board when the solders are subjected to heat of temperature higher than the melting point of the solders. A new bare chip or MCM is thereafter mounted on the printed circuit board by soldering input/output terminals of the new bare chip or MCM to the input/output pads which have previously received the input/output terminals of the old bare chip or MCM.
It is well known that part of the old solder still remains on the input/output pad on the printed circuit board even after the old bare chip or MCM is detached in the above-described exchanging operation. When a new bare chip or MCM is mounted on the printed circuit board, a new solder of a predetermined amount is added to the old solder remaining on the input/output pad. An excessive solder may induce a short between the adjacent input/output terminals.
In particular, the respective input/output pads are not expected to keep solders of a uniform amount remaining thereon. When a bare chip or MCM employs a plurality of input/output terminals such as a ball grid array (BGA) and a pin grid array (PGA), the bare chip or MCM may suffer from the solders of a varied height on the input/output pads. The input/output terminals of the new bare chip or MCM may in part fail to touch the surface of the input/output pads on the printed circuit board. No electric connection can thus be achieved partly.
In view of the above inconvenience, the solders remaining on the input/output pads should be wiped out before a new bare chip or MCM is mounted. For example, the printed circuit board along with the remaining bare chips or MCMs is subjected to another heat so as to melt the solders remaining on the input/output pads. Heat is sometimes repeatedly applied to the printed circuit board more than twice until the solders are completely wiped out. Repeated application of heat is supposed to damage and shorten the life of the printed circuit board, and bare chips or MCMs remaining on the printed circuit board. It sometimes deteriorates the wetness of the input/output pads to solders.
It is accordingly an object of the present invention to provide a printed circuit board capable of avoiding repeated application of heat even when an electronic component is exchanged.
According to the present invention, there is provided a printed circuit board unit comprising: a printed circuit board; an electronic component; a solder bump interposed between the printed circuit board and the electronic component so as to fix the electronic component to the printed circuit board; and an insulated film disposed between the printed circuit board and the electronic component so as to define a through hole for receiving the solder bump.
With the above structure, electric connection can reliably be established between the printed circuit board and the electronic component since the solder bump is allowed to penetrate through the through hole in the insulated film disposed between the printed circuit board and the electronic component. The insulated film is highly expected to contribute to detachment of the electronic component from the printed circuit board.
For example, the through hole may be designed to form a constriction in the solder bump between the printed circuit board and the electronic component. When the insulated film is brought away from the printed circuit board, the insulated film serves to tear the solder bump in two pieces at the constriction, so that the electronic component can easily be detached from the printed circuit board.
In this case, the insulated film is preferably superposed on the printed circuit board so as to form the constriction right on a conductive pad on the printed circuit board. Such constriction serves to allow the solder bump of a constant amount to remain on the conductive pad, so that a plurality of solder bumps of a uniform amount are allowed to remain on the corresponding conductive pads. The thinner the insulated film gets, the less the solder bump remains on the conductive pad.
Such a through hole, standing on the conductive pad so as to form the constriction in the solder bump, may have the inner peripheral size smaller than the outer peripheral size of the conductive pad. Such a through hole may serve to prevent the melting solder bump from flowing off the periphery of the surface of the conductive pad. In general, the conductive pad usually comprises a base conductive layer on a substrate of the printed circuit board, and a surface conductive layer superposed on the top surface of the base conductive layer. The surface conductive layer is designed to have a corrosion resistance higher than the base conductive layer. If the solder bump is prevented from flowing around the surface conductive layer so as to reach the base conductive layer in the above manner, it is possible to reliably avoid erosion of the base conductive layer such as a copper layer. The base conductive layer is prevented from getting thinner or smaller. The surface conductive layer may be a nickel layer.
When the electronic component is to be detached from the printed circuit board, the through hole may be displaced along the surface of the conductive pad. The solder bump should be kept at a melting temperature. The sliding movement of the insulated film along the surface of the printed circuit board serves to completely wipe out the melting solder bump from the conductive pad. The solder bump is torn apart. Moreover, the solder bump hardly remains on the surface of the conductive pad.
Furthermore, the inner surface of the through hole may be covered with a coating wet to the solder bump. In detaching the electronic component from the printed circuit board in this case, a relative movement may be caused between the conductive pad and the through hole while the solder bump is kept at a melting temperature. When the inner surface of the through hole moves across the solder bump, the melting solder bump is dragged by the coating having a higher wetness to the solder bump. The solder bump can be removed.
Furthermore, the thickness of the insulated film may be designed to correspond to the height of the solder bump on the printed circuit board. Such an insulated film serves to surround the solder bump, received on the conductive pad, within the inner wall of the through hole. In detaching the electronic component from the printed circuit board, a relative movement may be caused between the conductive pad and the through hole while the solder bump is kept at a melting temperature. The inner wall of the through hole is designed to completely wipe out the solder bump from the conductive pad. Moreover, the solder bump hardly remains on the surface of the conductive pad.